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Abstract:

A minute amount of fuel is injected into a combustion chamber during fuel
cut, and a cylinder pressure is detected when the minute amount of fuel
is being combusted. Then a combustion ratio is calculated based on the
detected cylinder pressure, and a determination crank angle, which is
used for determining a cetane number based on the combustion ratio, is
determined. The cetane number is determined based on the determination
crank angle.

Claims:

1-4. (canceled)

5. A cetane number determination apparatus for a diesel engine,
comprising: a cylinder pressure sensor that detects a pressure in a
cylinder of the diesel engine; a crank angle sensor that detects a
rotational position of a crankshaft of the diesel engine; and a control
device that calculates the combustion ratio at a threshold crank angle at
a combustion midpoint based on a first pressure in the cylinder at a
first crank angle at a combustion start point, a second pressure in the
cylinder at a second crank angle at a combustion end point, and a third
pressure in the cylinder at the threshold crank angle, and determines the
cetane number based on the combustion ratio at the threshold crank angle.

6. The cetane number determination device according to claim 5, wherein
the control device executes a control so that a minute amount of fuel is
injected into a combustion chamber during fuel cut or when a condition of
fuel cut is met, and the control device calculates the combustion ratio
at the threshold crank angle when the minute amount of the fuel is being
combusted.

7. The cetane number determination apparatus according to claim 5,
wherein: a second correlation map of the cetane number and the combustion
ratio at the threshold crank angle is stored in the control device; and
the control device calculates the combustion ratio at the threshold crank
angle, and determines the cetane number based on the calculated
combustion ratio at the threshold crank angle and the second correlation
map.

8. The cetane number determination device according to claim 7, wherein a
plurality of the second correlation maps that differ depending on a
temperature of a coolant in the diesel engine are stored in the control
device.

9-11. (canceled)

12. The method of determining a cetane number for a diesel engine,
comprising: detecting a first pressure in a cylinder at a first crank
angle at a combustion start point, a second pressure in the cylinder at a
second crank angle at a combustion end point, and a third pressure in the
cylinder at a threshold crank angle at a combustion midpoint; calculating
a combustion ratio at the threshold crank angle based on the first
pressure, the second pressure, and the third pressure; and determining
the cetane number based on a correlation map of the calculated combustion
ratio at the threshold crank angle and the cetane number.

13. The method of determining a cetane number for a diesel engine
according to claim 12, further comprising: detecting a temperature of a
coolant in the diesel engine; and selecting one correlation map that
corresponds to the detected temperature of the coolant among a plurality
of the correlation maps that differ depending on the temperature of the
coolant, and determining the cetane number based on the selected
correlation map.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a cetane number determination apparatus
for a fuel of a diesel engine and a method of determining a cetane number
of a fuel of a diesel engine.

[0003] 2. Description of the Related Art

[0004] It is necessary in a diesel engine, as well as a gasoline engine,
to handle various types of fuel in the optimal way. It is important, in
terms of reducing emission and improving drivability, to detect a cetane
number and perform the optimal fuel injection timing control.

[0005] The cetane number is a numerical index that indicates difficulty of
diesel knocking occurrence in a diesel engine (that is, knocking
resistance or an anti-knocking property), and that indicates
self-ignitability. The cetane number is correlated with a delay of
ignition. As the cetane number becomes larger, self-ignition is more
likely to occur, and knocking is less likely to occur.

[0006] Various technologies for determining the cetane number have been
proposed (for example, refer to Japanese Patent Application Publication
No. 2004-340026 (JP-A-2004-340026)).

[0007] The cetane number hardly influences the combustion characteristics
in a normal operation region, that is, when a normal fuel injection
amount is used. Therefore, it is not easy to accurately determine the
cetane number in the normal operation region.

[0008] Further, for example, as a method of determining the cetane number,
it is conceivable to set a threshold of a heat generation rate
dQ/dθ, thereby determining the ignition timing. However, this
requires a complicated process, such as a differentiation process, and
moreover, the determination result is likely to include a noise when this
method is used.

SUMMARY OF THE INVENTION

[0009] The invention provides a cetane number determination apparatus for
a fuel of a diesel engine, and a method of determining a cetane number of
a fuel of a diesel engine, in which it is possible to easily and
accurately determine the cetane number of the fuel injected into the
combustion chamber.

[0010] A first aspect of the invention relates to a cetane number
determination apparatus for a diesel engine. The cetane number
determination apparatus includes a control device that executes a control
so that a minute amount of fuel is injected into a combustion chamber
during fuel cut or when a condition of fuel cut is met, and determines a
cetane number based on a pressure in a cylinder detected when the minute
amount of the fuel is being combusted.

[0011] In this configuration, if fuel cut is being performed, that is, the
fuel injection is stopped, for example, when the vehicle is running on a
slope road, the minute amount of fuel that keeps the driver unaware of an
increase of an engine speed, or that hardly influences the torque, is
injected into the combustion chamber. Further, the fuel injection amount
and the detection error of the ignition timing are correlated with each
other. Thus, by using the minute amount of fuel for detecting the
ignition timing, it is possible to detect the ignition timing with higher
accuracy. Therefore, it is possible to accurately determine the cetane
number.

[0012] In the configuration as described above, the cetane number
determination apparatus may further include: a cylinder pressure sensor
that detects the pressure in the cylinder of the diesel engine; and a
crank angle sensor that detects a rotational position of a crankshaft of
the diesel engine. In the cetane number determination apparatus thus
configured, the control device may calculate a combustion ratio in the
cylinder with respect to a crank angle determined using the crank angle
sensor, based on the pressure in the cylinder, and may determine the
cetane number based on the combustion ratio.

[0013] In this configuration, the cetane number is determined based on the
combustion ratio in the cylinder with respect to the crank angle. Thus,
it is possible to more easily determine the cetane number.

[0014] A second aspect of the invention relates to a cetane number
determination apparatus for a diesel engine. The cetane number
determination apparatus includes a cylinder pressure sensor that detects
a pressure in a cylinder of the diesel engine; a crank angle sensor that
detects a rotational position of a crankshaft of the diesel engine; and a
control device that determines the cetane number based on a combustion
ratio in the cylinder at a threshold crank angle. In the cetane number
determination apparatus, the control device calculates the combustion
ratio at the threshold crank angle based on a first pressure in the
cylinder at a first crank angle at a combustion start point, a second
pressure in the cylinder at a second crank angle at a combustion end
point, and a third pressure in the cylinder at the threshold crank angle
at a combustion midpoint, and determines the cetane number based on the
combustion ratio at the threshold crank angle.

[0015] In this configuration, it is possible to determine the cetane
number using the values of the cylinder pressure taken at only three
combustion points, thereby substantially simplifying the cetane number
determination process.

[0016] A third aspect of the invention relates to a method of determining
a cetane number for a diesel engine. The method includes: injecting a
minute amount of fuel into a combustion chamber when it is determined
that fuel cut is being performed or when it is determined that a
condition of fuel cut is met, in the diesel engine; detecting a pressure
in a cylinder when the minute amount of the fuel is being combusted; and
determining the cetane number based on the pressure in the cylinder.

[0017] A fourth aspect of the invention relates to a method of determining
a cetane number for a diesel engine. The method includes: detecting a
first pressure in a cylinder at a first crank angle at a combustion start
point, a second pressure in the cylinder at a second crank angle at a
combustion end point, and a third pressure in the cylinder at a threshold
crank angle at a combustion midpoint; calculating a combustion ratio at
the threshold crank angle based on the first pressure, the second
pressure, and the third pressure; and determining the cetane number based
on a correlation map of the calculated combustion ratio at the threshold
crank angle and the cetane number.

[0018] According to these aspects of the invention, it is possible to
provide the cetane number determination apparatus for a fuel of a diesel
engine, and the method of determining a cetane number of a fuel of a
diesel engine, in which it is possible to easily and accurately determine
the cetane number of the fuel injected into the combustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The foregoing and further objects, features and advantages of the
invention will become apparent from the following description of
preferred embodiments with reference to the accompanying drawings,
wherein like numerals are used to represent like elements and wherein:

[0020] FIG. 1 shows a configuration of a diesel engine to which a cetane
number determination apparatus according to an embodiment of the
invention is applied;

[0021] FIG. 2 is a flowchart showing an example of a cetane number
determination process performed by an ECU;

[0022] FIG. 3 is a chart showing a relation between a fuel injection
amount and a detection error of an ignition timing;

[0023] FIG. 4 is a chart showing a correlation between a crank angle and
MFB;

[0024] FIG. 5 is a chart showing a correlation between the cetane number
and the ignition timing;

[0025] FIG. 6 is a flowchart showing another example of the cetane number
determination process performed by the ECU;

[0026] FIG. 7 is a chart explaining a method of determining the cetane
number; and

[0027] FIG. 8 is a chart showing a correlation between a cetane number
index and MFB.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Hereinafter, an embodiment of the invention will be described with
reference to the attached drawings.

[0029] FIG. 1 shows a configuration of a diesel engine to which a cetane
number determination apparatus according to an embodiment of the
invention is applied.

[0030] In an engine 10, a diesel oil is directly injected as a fuel from a
fuel injection valve 11 into a combustion chamber 12 in a compressed
state so as to naturally ignite air-fuel mixture.

[0031] A cylinder head 15 includes an intake port 13 and an exhaust port
14 connected to the corresponding combustion chamber 12. Further, the
cylinder head 15 further includes a valve operating mechanism 18 and a
fuel injection valve 11. The valve operating mechanism 18 includes an
intake valve 16 that opens and closes the intake port 13, and an exhaust
valve 17 that opens and closes the exhaust port 14. The fuel injection
valve 11 is disposed at a center of an upper portion of the combustion
chamber 12 so as to be positioned between the intake valve 16 and the
exhaust valve 17.

[0032] An intake pipe 20 is connected to the cylinder head 15 in a manner
such that the intake pipe 20 communicates with the intake port 13, and an
intake passage 19 is defined by the intake pipe 20 and the intake port
13. An air cleaner 21 is provided upstream of the intake pipe 20. The air
cleaner 21 functions to remove dust, etc., contained in the air so that
the clean air is supplied to the intake passage 19. A surge tank 22 is
formed in the intake pipe 20 at a position upstream of the intake valve
16, and an intake control valve 24 is provided in the intake pipe 21 at a
position between the intake valve 16 and the surge tank 22. The intake
control valve 24 is operated by an actuator 23 so as to open and close
the intake passage 19 at predetermined timings in accordance with the
opening and closing timings of the intake valve 16. When the engine 10
includes a plurality of intake ports 13 for each cylinder, the intake
control valve 24 may be independently provided for each intake port 13 so
as to independently open and close the intake port 13. Further, the
intake control valves 24 may be opened and closed on a
cylinder-by-cylinder basis. The intake control valve 24 and the actuator
23 have extremely good control response so that the intake control valve
24 is accurately opened and closed at desired timings in accordance with
the opening and closing timings of the intake valve 16.

[0033] An exhaust pipe 27 is connected to the cylinder head 15 in a manner
such that the exhaust pipe 27 communicates with the exhaust port 14, and
an exhaust passage 26 is defined by the exhaust pipe 27 and the exhaust
port 14. A catalyst 28 functions to purify pollutant produced by the
combustion of air-fuel mixture in the combustion chamber 12. The catalyst
28 is provided in the exhaust pipe 27.

[0034] Therefore, the intake air flows in the intake pipe 20 through the
air cleaner 21 and is supplied into the combustion chamber 12. The air
thus supplied into the combustion chamber 12 is mixed with the fuel
injected from the fuel injection valve 11 into the combustion chamber 12,
whereby the air-fuel mixture is produced. Then, the air-fuel mixture is
naturally ignited when a piston 29 is near a compression top dead center,
and thus, the air-fuel mixture is combusted. Exhaust gas, which is
produced by the combustion, flows in the exhaust pipe 27 through the
catalyst 28 and is discharged from the exhaust pipe 27 to the outside
air.

[0035] Various sensors as listed below are provided in order to detect the
operational conditions of the engine 10 and the vehicle in which the
engine 10 is mounted so that the ECU 25 accurately controls the amount
and timing of the fuel injection from the fuel injection valve 11 and
operation of the intake control valve 24. More specifidally, an
accelerator operation amount sensor 31 is provided so as to detect an
operation amount of an accelerator pedal 30 operated by a driver, and
output the detected operation amount to the ECU 25. Further, an intake
air temperature sensor 32 and an intake air pressure sensor 33 are
provided in a portion of the intake pipe 20 downstream of the intake
control valve 24. The intake air temperature sensor 32 detects an intake
air temperature Ts in the intake passage 19 and outputs the detected
temperature Ts to the ECU 25. The intake air pressure sensor 33
detects an intake air pressure in the intake passage 19 and outputs the
detected pressure to the ECU 25.

[0036] Further, a cylinder pressure sensor 40 is provided to detect the
pressure in the combustion chamber 12 of each cylinder.

[0037] Further, a crank angle sensor 37 is provided in a cylinder block 34
in which the piston 29 reciprocates. The crank angle sensor 37 detects
the rotational position of a crankshaft 36 to which the piston 29 is
connected through a connecting rod 35, that is, a crank angle phase, and
outputs the detection result to the ECU 25. The crank angle is determined
using the crank angle sensor 37.

[0038] The ECU 25 controls operation of the fuel injection valve 11, the
actuator 23, and the like based on detection signals output from the
accelerator operation amount sensor 31, the intake air temperature sensor
32, the intake air pressure sensor 33, the crank angle sensor 37, the
cylinder pressure sensor 40, and the like so that the engine 10 is
smoothly operated in accordance with a program that is preliminarily set.

[0039] Next, one example of a cetane number determination process
performed by the ECU 25 will be described with reference to FIGS. 2 to 5.
FIG. 2 is a flowchart showing one example of the cetane number
determination process performed by the ECU 25, and FIG. 3 is a chart
showing a relation between the fuel injection amount and the detection
error of ignition timing. Further, FIG. 4 is a chart showing a
correlation between the crank angle and MFB, and FIG. 5 is a chart
showing a correlation between the cetane number and the ignition timing.

[0040] As shown in FIG. 2, if fuel cut is being performed, that is, the
fuel injection from the fuel injection valve 11 is stopped, for example,
when the vehicle islunning on a slope road (downhill road) (step S1), the
ECU 25 controls the fuel injection valve 11 to inject a minute amount of
fuel into the combustion chamber 12 (step S2). The minute amount of fuel
signifies the amount of fuel that keeps the driver unaware of an increase
of an engine speed, or the amount of fuel that hardly influences the
torque. Further, the detection error of the ignition timing is a
difference between an actual ignition timing and an ignition timing
detected based on ΔPV.sup.κ determined based on a cylinder
pressure P, a volume V of the combustion chamber 12, and a specific heat
ratio κ, which will be described later. The fuel injection amount
and the detection error of the ignition timing are correlated with each
other, for example, as shown in FIG. 3. The correlation such as that
shown in FIG. 3 is experimentally obtained. As it can be understood from
the chart in FIG. 3, as the fuel injection amount becomes larger, the
detection error of the ignition timing when the ignition timing is
detected becomes larger. Therefore, the fuel injection amount is set to
the minute amount so that the detection accuracy of the ignition timing
is within the target accuracy range.

[0041] Next, the cylinder pressure P is detected using the cylinder
pressure sensor 40 (step S3). At the same time, the volume V of the
combustion chamber 12 is determined based on a crank angle θ
determined using the crank angle sensor 37, and the specific heat ratio
κ is also determined. Then, the value of PV.sup.κ is
calculated using the cylinder pressure P, the volume V of the combustion
chamber 12, and the specific heat ratio κ (step S4). PV.sup.κ
is a parameter that is substantially proportional to a heat generation
amount Q. Further, the value of PV.sup.κ is calculated, for
example, each time the crank angle θ changes by one degree.

[0042] Next, a combustion ratio MFB (θ), which is the ratio of
combustion in the cylinder with respect to the crank angle θ, is
calculated using the calculated value of PV.sup.κ. MFB is
represented by an equation 1 below, for example, when a combustion start
point is -4°, a combustion midpoint, at which the cetane number
should be determined, is θ, and a combustion end point is
30°.

[0043] Next, as shown in FIG. 4, the calculated value of MFB (θ) is
compared with a determination threshold (for example, 30%) (step S6) so
as to determine a crank angle θCN when the value of MFB
(θ) is equal to or more than the determination threshold (step S7).
Note that, as shown in FIG. 4, the cetane number and MFB are correlated
with each other. That is, as the cetane number becomes smaller, the crank
angle θCN becomes larger.

[0044] Next, the temperature of an engine coolant is determined (step S8).
A cetane number index CN is correlated with the crank angle
θCN when the value of MFB (θ) exceeds the determination
threshold (for example, 30%) as shown in FIG. 5. The correlation is
mapped in accordance with the temperature of the engine coolant, and the
cetane number index CN that corresponds to the crank angle θCN
determined in step S7 is determined based on the map (step S9).

[0045] Then, an injection timing control is performed so that the fuel is
injected at the optimal timing in accordance with the determined cetane
number (step S10).

[0046] As described above, according to the embodiment, MFB correlated
with the cetane number is used for determining the cetane number, and the
calculation of MFB does not involve any complicated process, such as a
differentiation process. Therefore, the accuracy of the result of the MFB
calculation is high, and thus, it is possible to obtain a high
signal-to-noise ratio (S/N ratio).

[0047] Next, another example of the cetane number determination process
performed by the ECU 25 will be described with reference to FIGS. 6 to 8.
FIG. 6 is a flowchart showing another example of the cetane number
determination process performed by the ECU 25, and FIG. 7 is a chart
explaining a method of determining the cetane number. Further, FIG. 8 is
a chart showing a correlation between the cetane number index and MFB.

[0048] As shown in FIG. 6, if fuel cut is being performed, that is, the
fuel injection from the fuel injection valve 11 is stopped, for example,
when the vehicle is running on a slope road (downhill road) (step S11),
the ECU 25 controls the fuel injection valve 11 to inject a minute amount
of fuel into the combustion chamber 12 (step S12). As described above,
the minute amount of fuel signifies the amount of fuel that keeps the
driver unaware of the increase of the engine speed, or the amount of fuel
that hardly influences the torque.

[0049] Next, the cylinder pressure P is detected using the cylinder
pressure sensor 40 (step S13). At the same time, the volume V of the
combustion chamber 12 is determined based on a crank angle θ
determined using the crank angle sensor 37, and the specific heat ratio
κ is also determined. Then, the values of PV.sup.κ are
calculated using the cylinder pressure P, the volume V of the combustion
chamber 12, and the specific heat ratio κ (step S14). As shown in
FIG. 7, the value of PV.sup.κ at the combustion start point
θstart, the value of PV.sup.κ at the combustion midpoint
θCN (for example, 8°) at which the cetane number should
be determined, and the value of PV.sup.κ at the combustion end
point θEND are calculated.

[0050] As it can be understood from FIG. 7, the combustion midpoint
θCN serves as the determination threshold.

[0051] Next, the combustion ratio MFB (θCN), which is the ratio
of combustion in the cylinder with respect to the crank angle.
θCN at the combustion midpoint at which the cetane number
should be determined, is calculated according to an equation 2 below,
using PV.sup.κ(θstart), PV.sup.κ(θCN),
and PV.sup.κ(θEND).

[0052] Next, the temperature of the coolant is read (step S16). The cetane
number index CN, the determination threshold θCN, and MFB
(θCN) are correlated with each other as shown in FIG. 8 (the
correlation varies depending on the temperature of the coolant), and the
correlation is preliminarily mapped. The cetane number index CN that
corresponds to MFB (θCN) determined in step S15 is determined
based on the map (step S17).

[0053] Then, the injection timing control is performed so that the fuel is
injected at the optimal timing in accordance with the determined cetane
number (step S18).

[0054] As described above, according to the embodiment, MFB
(θCN) is determined using PV.sup.κ(θstart),
PV.sup.κ(θCN), and PV.sup.κ(θEND), and
the cetane number is directly determined based on MFB (θCN)
thus determined. Accordingly, the processing load is further reduced, and
it is possible to determine the cetane number with higher accuracy.

[0055] In the embodiments described above, it is determined whether fuel
cut is being performed when the minute amount of fuel is injected into
the combustion chamber 12. However, it may be determined whether a
condition of fuel cut is met when a minute amount of fuel is injected
into a combustion chamber.